1. Field
The embodiments herein are directed to an optical device, an optical modulation method, and an optical transmitter.
2. Description of the Related Art
In recent years, the demand for the introduction of a next-generation 40-Gbps optical transmission system has been increasing with the increase in transmission traffic. Further, the next-generation 40-Gbps optical transmission system requires a transmission distance and a spectral efficiency equal to those of the conventional 10-Gbps system. Modulation methods such as RZ-DPSK (return to zero—differential phase shift keying) and CSRZ-DPSK (carrier-suppressed return-to-zero—DPSK) modulation methods are being actively researched and developed. These modulation methods are excellent in the tolerance to the optical signal to noise ratio (OSNR) and the nonlinear tolerance compared with the NRZ (no return to zero) modulation method that has been applied in the conventional system.
Of these methods, for example, an RZ-DQPSK (RZ-differential quadrature phase-shift keying) modulation method having a characteristic of a narrow spectrum (high spectral efficiency) is a candidate for the modulation method of the next-generation optical transmission system. FIG. 18 illustrates an example of the structure of an optical modulation device adopting the RZ-DQPSK modulation method of, for example, 40-Gbps.
The optical modulation device 100 illustrated in FIG. 18 has a DQPSK modulator 101 and an RZ modulator 102. The DQPSK modulator 101 has an outer Mach-Zehnder interferometer 103. An I arm and a Q arm included in the Mach-Zehnder interferometer 103 have inner Mach-Zehnder interferometers 104i and 104q, respectively. The inner Mach-Zehnder interferometers 104i and 104q each perform a binary phase modulation on the input light based on a 20-Gbps data signal. In each of the inner Mach-Zehnder interferometers 104i and 104q, an electrode is formed on the arm, and the data signal as a voltage signal is supplied to the electrode, whereby the input light is phase-modulated.
At this time, as the two 20-Gbps data signals input to the DQPSK modulator 101, signals whose waveforms are deteriorated by a preceding circuit are input. Therefore, the signals are waveform-shaped by using D flip-flops (DFFs) 106i and 106q. For example, two differential signals (pair of signals which are inverted with respect to each other) corresponding to the input 20 Gbps data signals are output as output data signals in synchronism with a clock signal from a 20-GHz clock signal source 110.
Then, the output data signals from the DFFs 106i and 106q are amplified by driver amplifiers 107i and 107q, respectively, and supplied to the electrodes formed on the arms of the inner Mach-Zehnder interferometers 104i and 104q as driving voltage signals of the DQPSK modulator 101. Consequently, phase-modulated light is output from each of the inner Mach-Zehnder interferometers 104i and 104q. 
Reference sign 108q represents a phase shifter that phase-shifts the light phase-modulated by the inner Mach-Zehnder interferometer 104q, by π/2. The outer Mach-Zehnder interferometer 103 splits the continuous light from a laser diode (LD) 105 so as to be supplied to the inner Mach-Zehnder interferometers 104i and 104q, multiplexes the lights phase-modulated by the inner Mach-Zehnder interferometers 104i and 104q, and outputs the multiplexed light as a DQPSK-modulated optical signal.
The RZ modulator 102 RZ-modulates the DQPSK optical signal from the DQPSK modulator 101 based on the clock signal input from the clock signal source 110. In this case, a 20-GHz clock signal is used as the driving signal of the RZ modulator 102, and the DQPSK optical signal input to the RZ modulator 102 is pulsed by the 20-GHz clock signal and output as an RZ-DQPSK-modulated optical signal. Reference sign 109 represents a driver amplifier that amplifies the 20-GHz clock signal and supplies it to the RZ modulator 102 as the driving signal.